1. Field of the Invention
The present invention relates to burglar detection devices in general and to ultrasonic intrusion alarms in particular.
2. Description of the Prior Art
Ultrasonic intrusion alarm systems and their attendant problems are well known in the prior art. One of the major difficulties associated therewith is how to achieve a system sensitivity adequate to protect more than the smallest space without incurring an unacceptable number of false alarms. Typically, as the system sensitivity is increased to extend the coverage, the system becomes increasingly vulnearable to extraneous signals such as those caused by air turbulence, sonic type background noise and electrical interference from power lines.
Even when an optimum sensitivity setting is obtained, changes in the environment such as the repositioning of furniture or humidity changes can radically effect the overall system sensitivity.
The inclusion of automatic gain control circuitry, at least where constant transmitted frequency signals are employed, may actually aggravate the problem. This is because a slight change in the position of the furniture, for instance, might cause the multiplicity of reflected signals comprising the received signal which might have formally arrived at the receiver in phase to add, to arrive out of phase to cancel. This of course, may cause wild fluctuation in the system sensitivity set by the automatic gain control circuitry.
Typical of the prior art, ultrasonic intrusion alarm systems which address the noise problem are two U.S. Pat. Nos. 3,781,859 and 3,721,972, both issued to Albert L. Hermans. The former patent also illustrates the use of special purpose filters for removing electrical power, radio and lightning caused interference as well as special circuitry for minimizing false alarms caused by air turbulence.
The latter patent illustrates the use of traditional ultrasonic transmitters and receivers externally mounted around an enclosed space which is to be protected from intrusion. In addition, one receiver is mounted inside the enclosed space so as to be substantially isolated from the transmitters by the walls enclosing the space.
This novel feature detects entry into the space by detecting the opening of a door or a window which completes a sonic path actuating the alarm.
This patent further recognizes another problem associated with ultrasonic intrusion alarms, namely, that when a multiplicity of remotely mounted transmitters and/or receivers are employed to increase the coverage of the system or to protect remote locations, the system becomes increasingly susceptible to tampering.